1. Field of the Invention
The present invention relates to a wire and cable for use in a robot such as an industrial robot, and particularly to a wire and cable which have both high electrical conductivity and excellent durability.
2. Description of the Related Art
In the industrial robot, an arm thereof moves in a complicated manner and therefore, cables for supplying electric power and transmitting electric signals to a means for processing such as a welding head attached to a distal portion of the arm are frequently bent at a joint of the arm.
In the conventional industrial robot, such a cable is manufactured by using an annealed copper wire. A plurality of annealed copper wires are intertwisted to form a twisted wire, which is coated with coating insulation member to form a coated wire and, a plurality of coated wires are gathered together and covered by a sheath to obtain a movable cable for use in the industrial robot.
In a case where a high durability of the cable is required, a wire made of high durable copper alloy such as Cu alloy containing Sn is used in the cable. The wire made of high durable copper alloy has a durability of five times as large as that of the annealed copper wire.
However, the electrical conductivity and the tensile strengh are generally in a reciprocal relationship and therefore, when a concentration of metal element added to copper is increased, then the conductivity is lowered while the tensile strengh is increased. For example, the pure copper has a conductivity of 100% IACS (International Annealed Copper Standard) which is so high as to be in the second place of that of silver. Whereas tensile strengh of the pure copper is lower than 40 Kg/mm.sup.2. On the other hand, a typical beryllium copper alloy for spring has a conductivity of lower than 30% IACS while a tensile strengh of the beryllium copper alloy is greater than 100 Kg/mm.sup.2.
In the above-mentioned Cu alloy containing Sn which have been used in trolley wire etc, as Sn concentration increases, then mechanical strengh is improved whereas the conductivity is lowered. For example, the conductivity of the copper alloy containing 0.3% Sn is 70% IACS and, that of the copper alloy containing 0.6% Sn becomes 50 to 60% IACS. In a case where the wire is made of material having lower conductivity, the diameter of the wire, of course, must be increased in order to lower the Joule heat. However, this makes it impossible to meet the requirement that the cable is made thinner and lighter to improve mobility of the arm of the robot.